1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method of operating a relay station in a wireless communication system.
2. Related Art
Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. In general, the wireless communication system is a multiple access system capable of supporting communication with multi-users by sharing available radio resources. Examples of the radio resource include a time, a frequency, a code, transmit power, etc. Examples of the multiple access system include a time division multiple access (TDMA) system, a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, etc. The radio resource is a time in the TDMA system, a frequency in the FDMA system, a code in the CDMA system, and a subcarrier and a time in the OFDMA system. A wireless communication system is a system supporting bidirectional communication. The bidirectional communication can be performed by using a time division duplex (TDD) mode, a frequency division duplex (FDD) mode, a half-frequency division duplex (H-FDD) mode, etc. The TDD mode uses a time resource to identify uplink transmission and downlink transmission. The FDD mode uses a frequency resource to identify uplink transmission and downlink transmission. The H-FDD mode uses a combination of a time resource and a frequency resource to identify uplink transmission and downlink transmission.
The wireless communication system includes a base station (BS) providing a service to a specific region (i.e., a cell). According to a characteristic of a wireless transmission technology, changes in a wireless environment have an effect on quality of signal transmitted. In particular, due to various factors in a surrounding environment, such as scatters, movement of a mobile station (MS), etc., a wireless channel changes over time. In addition, there is a restriction in terms of distance since reception power is rapidly decreased in proportion to a distance between wireless communication entities. Therefore, in general, the MS can communicate with the BS when the MS is located within the coverage of the BS. As such, due to several factors such as the scatters, a movement speed of the MS, a distance between transmission and reception, etc., a maximum transfer rate, a throughput of an intra-cell user, and a throughput of a whole cell are decreased between the BS and the MS. For example, when the MS is located in a cell boundary or when an obstacle such as a building exists between the MS and the BS, communication quality between the MS and the BS may not be satisfactory.
As an effort to overcome the aforementioned problem, several techniques are introduced to compensate for deterioration of signals transmitted between the BS and the MS, thereby obtaining a maximum transfer rate, throughput improvement, coverage expansion, etc. For this purpose, a wireless communication system may employ a relay station (RS). The RS can expand the coverage of the BS, and can improve a cell throughput.
According to functions of the RS, the RS can be classified into several types as follows.
TABLE 1Pico/L1L2L3FemtofunctionRelayRelayRelayCellRF functionXXXXCoder/Decoder and CRC—XXXHARQ—XXXMultiplex & Demultiplex—XXXof MAC SDUPriority(Qos) handling—XXXScheduling—XXXOuter ARQ—(X)XX(Re)-Segmentation and—(X)XXconcatenationHeader———Xcompression(ROHC)Reordering of lower———Xlayer SDUsIn-sequence delivery of———upper layer PDUsDuplicate detection of———Xlower layer SDUsCiphering———XSystem information——XXbroadcastRRC Connection set-up——XXand maintenanceRadio Bearers set-up———Xand maintenanceMobility function——XMBMS services control———XPaging———XQoS management——(X)XUE measurement——(X)Xreporting and controlthe reportingNAS signalling handling———X
Although the RS is classified into an L1 relay, an L2 relay, and an L3 relay in Table 1, this is for exemplary purposes only. The above classification is achieved according to a broad characteristic of the L1, L2, and L3 relays, and the terminology thereof is not limited thereto. By reference, Table 1 also provides a function of a femto cell or a pico cell. It is assumed that the femto cell or the pico cell supports all functions exemplified in Table 1. The L1 relay is an RS having an amplify and forward (AF) function as well as some additional functions. The L1 relay amplifies a signal received from a BS or an MS and delivers the amplified signal to the MS or the BS. The L2 relay is an RS having a decoding and forward (DF) function as well as a scheduling function. The L2 relay restores information by performing demodulation and decoding on a signal received from the BS or the MS, generates a signal by performing coding and modulation, and then delivers the generated signal to the MS or the BS. The L3 relay is an RS having a configuration similar to one cell. The L3 relay has the functions of the L2 relay and supports call access, release, and mobility functions.
The RS can transmit or receive data by using a radio resource. The radio resource that can be used by the RS includes a time resource, a frequency resource, a spatial resource, etc. The time resource is expressed by a subframe, a symbol, a slot, etc. The frequency resource is expressed by a subcarrier, a resource block, a component carrier, etc. The spatial resource is expressed by spatial multiplexing, an antenna, etc. Such a radio resource may be used in a dedicated or shared manner between the BS and the RS or between the RS and the MS.
The RS is a recently introduced concept, and has to support an MS devised without consideration of the RS. For example, although the RS is not considered in the long term evolution (LTE) standard, the RS has to support not only an MS conforming to the LTE-advance standard but also an MS conforming to the LTE standard.